Proceedings of the Institution of Mechanical Engineers,Part C: Journal of Mechanical Engineering Science

Following an earlier comparative investigation of the tensile fatigue response, in terms of stress versus log life curves, of three epoxy-resin-based laminates of 0/90 lay-up and reinforced with carbon, glass and Kevlar-49 fibres, the authors now present a study of the accumulation of fatigue damage in terms of the residual strengths of the fatigued laminates. A stress-independent parametric relationship between normalized stress and normalized number of cycles is found to describe the behaviour of all three composites, suggesting a single major mode of fatigue damage despite the widely different mechanical characteristics of the different materials. The model is a two-parameter power law model, derived from a consideration of the appropriateness of various curve-fitting methods, and permits the incorporation of all modes of damage accumulation, from gentle ‘wear-out’ to ‘sudden death’ simply by the adjustment of this pair of power law exponents.

The spectral analysis of surface texture data is extended so as to include the longer wavelength ‘waviness’ features in addition to the ‘roughness’ aspects. This results in the identification of an underlying form in the spectral estimates of engineering surfaces whose complete description requires only two parameters. Also, by showing that the spectral estimates converge to a final steady value at the longer wavelengths, the need to select and apply a standard cut-off filter is avoided.

A method is presented for the calculation of two-dimensional, incompressible, transitional boundary layers under small pressure gradient and moderate freestream turbulence conditions. Established integral techniques are used in conjunction with an intermittency weighted model of the transitional boundary layer, and empirical correlations are used to predict the onset and length of the transition region. The only input data required to compute the entire unseparated boundary layer are the ambient pressure and temperature, the freestream turbulence level and the freestream velocity distribution in a power law, or a polynomial form. Alternatively, the freestream velocity can be input in tabular form as a function of x.

The computed integral parameters and mean velocity profiles are seen to compare favourably with present and other published experimental data.

The clearances in highly loaded non-Hertzian contacts can be calculated directly from the dry contact pressure distribution. This note presents a method of extending the analysis into less highly loaded regions. It is shown that the method accurately predicts the clearance over much of the transition zone for Hertzian contacts and its use in a non-Hertzian situation is illustrated using the contact between a rigid cylinder and an elastomer-lined surface as an example.

Techniques for modelling electrochemical machining are briefly reviewed before concentrating on the boundary element method for computing the shape of tool profiles. The model described caters for problems which can be represented geometrically in two dimensions; it is iterative in nature and uses the approximate cos θ method to initiate the procedure. Subsequent iterations employ one of three formulations developed for correcting the tool profile to obtain workpiece equilibrium. Linear and quadratic isoparametric elements have been used and their relative accuracy is assessed. Special emphasis is placed on the design of tools expected to need sharp profile discontinuities and the merits of specifying different boundary conditions on the known work surface are examined.

Experimental investigations of the lubricant film thickness generated in elastohydrodynamic elliptical contacts have been undertaken since the early 1960s. The majority of these studies has been concerned with circular or near circular contact situations, although a wide range of geometries in which lubricant entrainment was directed along the minor axis of the contact ellipse has also been considered. The information available on lubricant film thickness in geometrical conditions where lubricant entrainment was aligned with the major axis of the contact ellipse has, however, been severely limited.

The experimental investigation described in this paper is therefore concerned with the measurement of lubricant film thickness in the unusual geometrical conditions recently analysed theoretically by the authors (1, 2). The measurements were made with the aid of a twin disc machine for geometries ranging from a radius ratio of unity down to a value of 0.112. The experimental apparatus is described and details are given of the special test discs which were manufactured to produce such geometries. The capacitance measurement technique adopted is detailed along with the numerical model developed to allow the measured values of inter-disc capacitance to be interpreted in terms of the lubricant film thickness.

The lubricant film thickness developed in four geometrical situations was investigated at four rotational speeds and five loads. This allowed a comparison to be made with the film thickness values predicted by recent elastohydrodynamic theory for changes in both speed and load. The film thickness deduced from the capacitance measurements was considered to be representative of that found at the centre of contact, and good agreement was found between experiment and all aspects of the theoretical predictions.

The existence of a coherent film of lubricant between highly loaded machine elements has been recognized for many years. Over this period of time measurements of film thickness have gone hand in hand with theoretical analyses in the field now known as elastohydrodynamic lubrication. The experimental techniques of capacitance, electrical resistance and X-ray measurement have been supplemented by the use of optical interferometry while the analytical expressions obtained with the use of elegant simplifications have been superseded by those developed from extensive and comprehensive computational procedures.

These developments in experimental techniques have yielded a substantial number of measurements of both minimum and central film thickness. Likewise, the advent of the digital computer has allowed the derivation of a large number of solutions to the problem of elastohydrodynamic lubrication of concentrated contacts. All these results, covering a wide range of geometrical conditions, are to be found in the literature, yet little attempt appears to have been made to assemble a representative set of experimental data to permit a detailed evaluation of the theoretical formulae for elliptical contacts.

The second part of this paper therefore considers the correlation between a number of experimental studies covering a wide range of operating conditions and geometries, and the predictions of recent elastohydrodynamic theory. Some of the important aspects of each set of experimental results are then considered and examples are provided which illustrate the following points:

1.

Good estimates of lubricant film thickness may be obtained from the theoretical expressions recently derived, even when the dimensionless parameters involved are outside the ranges considered in the derivation of the formulae.

On the whole there is good agreement between experiment and theory, while the general trend of the results indicates that theoretical predictions may underestimate the minimum film thickness by about 10 per cent and the central film thickness by about 25 per cent. This measure of agreement is quite remarkable when the extreme difficulty of interpreting the magnitudes of effective and very thin mean film thicknesses between machined components in various forms of experimental equipment is considered.